The complex differentiation program undergone by muscle cells includes the assembly of myofibrils that involves the timely interactions of many proteins. Understanding the protein-protein interactions involved is critical for our understanding of muscle structure and function. Besides the thin and thick filaments, all striated muscles contain a third filament implicated in myofibrillar elasticity. In vertebrates, this elastic filament is composed of the protein titin, and in insects the protein projectin is considered a homologue of titin, as the two proteins share a similar modular structure. Mutations in titin have been associated with two human dystrophies: a form of hypertrophic cardiomyopathy and tibial muscular dystrophy whereas homozygous mutant alleles of Drosophila projectin are embryonic lethal. Therefore, both the human and Drosophila mutant phenotypes indicate that the elastic function is essential for muscle physiology. Analysis of elastic proteins in both vertebrate and invertebrate muscles is, therefore, critical for our understanding of muscle function, and Drosophila, with its genetic protocols, is an excellent model system for such studies. The long-term objective of the P.l.'s laboratory is to decipher the roles of projectin in the formation and function of myofibrils. Projectin is a giant protein and its conformation allows it to span the entire Z-l region within the sarcomere. The first objective of this proposal is to assess length changes associated with muscle stretch (aim #1), and to address the interactions of different projectin domains with other myofibrillar proteins (aim #2). In the second aim, the P.I. will address the in vivo binding of GFP-projectin domain fusion proteins in pupae and adults of wild type and mutants. In aim #3 the P.I. will use an RNAi approach to inhibit the production of projectin and will use these "projectin-null" stocks to evaluate the importance of projectin in myofibril assembly and function. The information obtained will advance the general knowledge on elastic filaments and their role within the sarcomere. The proposed research will greatly impact undergraduate education at the College of Charleston as six to seven undergraduate students will be directly involved in research projects. The main goal is to expose them to the research environment, and to teach them the scientific methods including designing, preparing, performing and analyzing their own experiments.